1. Technical Field
The embodiments described herein are related to wireless communication and in particular to systems and methods for medical sensors that can sense a plurality of vital signs and that can provide imaging within the body.
2. Related Art
Remote monitoring in health and medical applications is becoming more and more common and may hold the key to reversing the health spending curve in the the United States and other nations. Remote monitoring, m-health, wireless health, etc., applications may also lead to improved care and better preventative care. There are numerous conditions, events, situations, etc., where adequate remote monitoring could prevent unwarranted trips to the hospital, emergency room, doctor's office, etc., and that could allow patients to stay at home instead of being in a hospital.
For example, fetal monitoring systems could allow an expecting mother to remain at home while the babies heart rate and mother's contraction are being monitored in certain situations, rather than the mother having to go to the hospital for such monitoring when it is really not necessary. Patients can be better monitored after discharge after surgery or a traumatic event like a heart attack, brain injury, or stroke. Thus, preventing re-admittance or trips to the hospital that are not necessary and also identifying conditions or behaviors that do necessitate a trip to the hospital or doctor's office.
Micropower Impulse Radar systems have also been developed that can enable low power, low cost sensors and imaging devices. Conventional radar sends out short bursts of single-frequency (narrowband) electromagnetic energy in the microwave frequency range. Other radars step through multiple (wideband) frequencies to obtain more information about a scene. An impulse, or ultrawide-band, radar such as MIR sends individual pulses that contain energy over a very wide band of frequencies. The shorter the pulse, the wider the band, thereby generating even greater information about reflected objects. Because the pulse is so short, very little power is needed to generate the signal. MIR is unique because it inexpensively generates and detects very fast (subnanosecond) pulses. The drawback of using short, low-power pulses is that less energy can be measured on the radar returns. This problem can be solved by transmitting many pulses rapidly and averaging all returns.
The advantages of producing and detecting very brief radar impulses are considerable:                The target echoes return much information. With short pulses, the system operates across a wide band of frequencies, giving high resolution and accuracy. The system is also less susceptible to interference from other radars.        Battery current is drawn only during the short time the system is pulsed, so power requirements are extremely low (microamperes). One type of MIR conventional unit operates for several years on two AA batteries.        The microwave power associated with pulsed transmission is exceedingly low (averaging tens of microwatts) and is medically safe. MIR emits less than one-millionth the power of a cellular telephone.        
FIG. 1 is a diagram illustrating an example, conventional MIR circuit developed by Lawrence Livermore Laboratories. The circuit of FIG. 1 can be used in a MIR motion sensor. In the MIR motion sensor, a transmitting antenna radiates a pulse that is about 0.2 nanoseconds long. Reflections from targets return a complex series of echoes to the receiving antenna. The return signal is sampled at one range-gate time by an impulse receiver containing a voltage sampler along with an averaging circuit and amplifier. The detector listens at the appropriate time for an echo. For an object about 3 m from the MIR, the sampled gate at 20 nanoseconds after transmission would just capture it.
Because the wavelength of MIR signals in a conventional system in air is currently about 15 cm, objects can easily be detected that are of about that size or larger at distances of about 15 cm or greater. Distorted, low-amplitude reflections of the transmitted pulse are picked up by the receiving antenna in the time it takes for light to travel from the MIR to the object and back again.
The operating principle of MIR motion sensors illustrated in FIG. 1 is based on the relatively straightforward principle of range gating. In looking for the return signals, MIR samples only those signals occurring in a narrow time window after each transmitted pulse, called a range gate. If we choose a delay time after each transmitted pulse corresponding to a range in space, then we can open the receiver “gate” after that delay and close it an instant later. In this way, we avoid receiving unwanted signals.
The MIR receiver has a very fast sampler that measures only one delay time or range gate per transmitted pulse, as shown in FIG. 2A. In fact, circuitry can be used that is similar to the transmit impulse generator for this range-gated measurement, another unique feature of our device. Only those return pulses within the small range gate corresponding to a fixed distance from device to target-are measured. The gate width (the sampling time) is always fixed based on the length of the pulse; but the delay time (the range) is adjustable, as is the detection sensitivity. Averaging thousands of pulses improves the signal-to-noise ratio for a single measurement; i.e., noise is reduced, which increases sensitivity. A selected threshold on the averaged signal senses any motion and can trigger a switch, such as an alarm.
A noise source is intentionally added to the timing of the circuitry in FIG. 1 so that the amount of time between pulses varies randomly around 2 MHz. There are three reasons for randomizing the pulse repetition rate and averaging thousands of samples at those random times. First, interference from radio and TV station harmonics can trigger false alarms; but with randomizing, interference is effectively averaged to zero. Second, multiple MIR units can be activated in one vicinity without interfering with each other if the operation of each unit is randomly coded and unique. Each unit creates a pattern recognizable only by the originating MIR. Third, randomizing spreads the sensor's emission spectrum so the MIR signals resemble background noise, which is difficult for other sensors to detect. Emissions from an MIR sensor are virtually undetectable with a conventional radio-frequency receiver and antenna only 3 m away. In other words, randomizing makes the MIR stealthy.
More sophisticated MIR sensors, such as our MIR Rangefinder, cycle through many range gates. As shown in FIG. 2B, the delay time is swept, or varied, slowly with each received pulse (about 40 sweeps per second) to effectively fill in the detection bubble with a continuous trace of radar information. In essence, we are taking samples at different times, thus different distances, away from the device. The result is an “equivalent-time” record of all return pulses that can be correlated to object distance. The equivalent-time echo pattern exactly matches the original “real-time” pattern, except that it occurs on a time scale slowed by 106.
Referring to FIG. 2A, following an impulse transmitted by MIR, a range gate opens briefly after a fixed delay time to sample the received radar echoes. In FIG. 3B it can be seen that to obtain a more complete record of returns for more sophisticated applications, we sweep the range delay over various delay times to obtain target information at different distances. The radar signal has then been effectively slowed down by about a factor of 1 million to get an “equivalent-time” record of radar returns that can be correlated to object distances.
As conventional MIR technology has evolved, a unique combination of features has resulted. Although certain specifications-signal strength, operating range, and directionality can vary depending on the type of system and its intended purpose, the following features are common to most conventional units:                Low cost, using off-the-shelf components.        Very small size (circuit board is about 4 cm2).        Excellent signal penetration through most low-conductivity materials, so it is able to “see through” walls, concrete, and other baniers, including human tissue.        A sharply defined and adjustable range of operation, which reduces false alarms.        Long battery life, typically several years, because of micropower operation.        Simultaneous operation of many units without interference.        Randomized emissions, making the sensor difficult to detect.        
Current MIR prototype units at LLNL are made with low-cost, discrete components. In the planning stages are single chips-application-specific integrated circuits (ASICs)-that will replace most of the discrete parts and result in even lower co t and smaller size. One limitation is that the penetration of MIR signals through a material decreases as that material's electrical conductivity increases. Thus, the MIR technology opens up many possible low-cost sensor systems for motion detection or proximity, distance measurement, microwave image formation, or even communications. For example, in some cases it has advantages over many kinds of conventional proximity and motion sensors, such as passive infrared (heat sensors), active beam-interruption infrared, ultrasound, seismic, and microwave Doppler devices.
Many of these sensors are adversely affected by temperature, weather, and other environmental conditions, making them prone to false alarms. Passive infrared sensors can be triggered by light and heat, and their detection range is not well defined. Even a thin sheet of paper blocks both infrared and ultrasound signals. Similarly, ultrasound motion and Doppler microwave sensors interfere with one another when several units are co-located. Without range gates, these sensors can trigger as easily on distant objects as on nearby insects. They can also have limited material penetration, detectable emissions, and expensive components. MIR technology provides an attractive alternative to these devices.
Further, a conventional MIR's average emission level is about a microwatt-about 3 order of magnitude lower than most international standard for continuous human exposure to microwave. Thus, MIR is a medically harmless diagnostic tool. This can enable sensors that can remotely measure human vital signs, without interfering with computer, digital watches, FM radio, or television.
For example, a MIR heart monitor can measure muscle contractions (response of the heart) rather than the electrical impulse (stimuli) measured with an electrocardiogram (EKG). FIG. 3 shows the output waveform of a prototype heart monitor compared to that obtained from a standard EKG. The MIR output is complex and rich in detailed information. As a medical monitor, a very small MIR unit built into a single chip could substitute for a stethoscope.
A portable device can then be developed that could be worn inside clothing so an individual's vital sign can be relayed from a remote location to a medical office or hospital.
An MIR-based breathing monitor can also be developed, see the ouput waveforms in FIG. 4 that does not have to make contact with a person's body. Rather, such a monitor could operate through a mattress, wall, or other barriers. The detection of breathing motion can be a valuable asset in hospitals and homes, could guard against sudden-infant-death syndrome, and might be used by people with breathing disorders such as sleep apnea, in which the affected individual occasionally stops breathing.
Additional potential medical devices that can take advantage of MIR technology include speech-sensing devices and a polygraph sensor. Devices for the blind could warn of obstacles and variations in terrain and help to train individuals in using canes.
One problem with conventional MIR technology is that it is still too high power to really enable a wide range of remote medical monitoring applications. In many instances, a remote sensor would be very small, very light weight, very low costs, and likely a throw away device. Moreover, there currently is not system that integrates MIR medical and imaging sensor data with other vital sign data such as temperature.